Electron multiplier



Nov. P. T. FARNSWORTH E? AL, 2,179,996

ELECTRON MULTIPLIER Filed NOV. 9, 1936 OUTPUT 0:32 :72? L/NPUT a9 a/ '3 ll 5g 3 Mg! 4 A 2 I OU TPUT INVEN TOR. PI-I/LO T. FARNSWO/PT H R/CHARD L. SNYDER Maw A TTORNEYS.

Patented Nov. 14, 1939 UNIEDSTATES PATENT OFFICE ELECTRON MULTIPLIER Philo T. Farnsworth, Springfield Township, Montgomery County, Pa., and Richard L. Snyder, Borough of Glasboro, N. J., assignors to Farnsworth Television & Radio Corporation, Dover, Del., a corporation of Delaware Application November 9, 1936, Serial No. 109,934-

embodiments are dealt with in the Farnsworth 13 Claims.

Our invention relates to means for electron multiplication, and particularly to multipliers utilizing an electron drift along continuous secondarily emissive surfaces.

5 Among the objects of our invention are:

provide more efiicient means for electron multiplication; to provide high gain directand alternating-current multipliers; to provide multiplying units which are free from the necessity for servicing at frequent intervals; to provide multiplying units so compact that they may be installed in cables or other places where space is limited; to provide multipliers which are operable upon local battery supply, or which may be fed through signal-carrying lines; multipliers having a low threshold point; to

to provide provide multipliers wherein the degree of multiplication may be varied over wide limits; to provide means for creating an electron drift,

whereby successive electron impacts may occur at points of increasingly higher potential along continuous secondarily emissive surfaces; and to provide electron drift in an electron multiplier to minimize space charge limitations therein.

7 'Our invention possesses numerous other objects and features of advantage, some of which,

together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing our novel method. is therefore to be understood that our method is applicable to other apparatus, and that we do not limit ourselves, in any way, to the apparatus of the present applicaiton, as we may adopt various other apparatus embodiments, utilizing claims.

In the drawing:

the method, within the scope of the appended Figure 1 is a partially sectional view .of our multiplier connected schematically for alternat- 40 ing-current energization.

Figure 2 is a schematic circuit diagram of a multiplier embodiment particularly suited to direct-current energization.

Figure 3 is a partially sectional, schematic along line 46 of Figure 3.

view of the embodiment shown in Figure 2.

Figure 4 is a partially sectional view taken The theory of electron multiplication by successive impacts of a modulated number of electrons upon secondarily emis'sive surfaces is well known since the development of the art and publication thereof by Farnsworth and his associates.

The present application is particularly concerned with means for producing an electron drift along a continuous surface, and related andSnyder application entitled Repeater, filed October 31, 1936, Serial No. 108,568, since matured into United States Patent No. 2,143,146, issued January 10, 1939.

The invention may be better understood by reference to the drawing.

In Figure 1 is shown one of our multiplier tubes schematically connected for operation, to be driven by alternating current. The tube comprises an evacuated, sealed, cylindrical envelope l of glass or other insulating material, having a radially extended neck 2 at one end, the neck terminating in a conventional press 3 on a reentrant stem 3. Through press 3 are sealed a pair of connecting leads 5 supporting a cathode, here shown as a filament 6, and a lead l which supports a grid 9 in position between filament 6 and the main body of the tube.

A grounded battery l connected to leads heats the filament 6. The signal input is applied to grid 9 through an input lead H and condenser l2.

The end of envelope l opposite neck 2 is terminated by a conventional press l3 on a reentrant stem it,

bearing a cathode support l5 sealed through sistor 26. carried to a radio-frequency ground 21.

The negative side of battery 25 is Cylindrical cathode Z9 is held symmetrically about anode 20 by supporting leads I5 and aperture 30, in line with filament 6, is provided through the wall of cathode 29.

A master oscillator 35 is connected through a variable condenser 32 and lead I6 to the cathode 29. A path to ground from lead I6 is made available through a choke coil 3%, resistor 35, and

battery 36.

The output lead 39 is fed through a condenser 31 tied to the multiplier side of resistor 35. The battery 36 maintains a constant positive component; upon cathode 29 of a few volts above ground potential.

A solenoid 33 wound about envelope I is provided, and is energized by a battery 38 to set up a magnetic field longitudinally of the envelope I.

In operation, filament 6 emits electrons when them pass through aperture 39, whilethe re mainder are collected on the cathode 29.

Those electrons passing through aperture 39 are accelerated toward anode 20 by the positive potential thereon, which is preferably of the order of 500 volts above ground, and is controllable by the variable resistor 29.

Relatively few of the accelerated electrons will be collected on the anode, due to its small superficial area and the deflection caused by the longitudinal magnetic field of solenoid 33. The

majority will pass by and continue to travel toward the opposite wall of cathode 29, being decelerated after passing the anode in the same degree as their initial acceleration.

The cathode potential has. during this time, become increasingly positive under excitation from the oscillator 3|, and this added potential causes the electrons to strike with sufficient velocity to knock out a number of secondary electrons. The interior of cathode 29 is coated, for greatest emciency, with a highly emissive material. caesium on an oxidized silver surface being a preferred form. These secondaries are attracted ,toward the anode, which they in most part pass,

impact the opposite cathode wall, again aided by the changing potential from the oscillator 3|. knock off more secondaries, and repeat. This process would continue, with an increasing cloud of electrons passing back and forth, until the space charge caused an equilibrium condition near the input stream were it not for our method of causing an electron drift, whereby the electron cloud is moved alongthe tube toward the collecting plate l9.

This drift is produced by the potential drop along anode wire 20, cooperating with the alternating current on cathode 29. Since this wire has a high resistance, and the voltage of battery 25 is high, a large gradient exists, with the points of higher positive potential near the collecting plate l9, and the lower potential at the end of anode 20 adjacent spring 2|.

Hence, each electron is attracted toward the anode in a path curving toward the higher potential points; successive passages bring the electron to the collecting plate, accomplishing the return circuit to the cathode through the radio-frequency ground 21, battery 36, resistor 35, choke coil 34 and lead l6. From the potential drop across the resistor 35, the useful output of the tube is obtained, through blocking condenser 31 and output lead 39.

It is apparent that in order to insure the numerous passages across the cathode necessary to obtain high multiplication, the frequency of the oscillator 3| must be properly co-ordinated with the dimensions of the cathode and the potential of battery 25, since polarity reversals of the oath-- ode must be so timed as to supply the electrons with suflicient impact velocity, and yet permit the secondaries to escape toward the anode. The electrons will, however, get slightly out of phase on each passage, since the added velocity approaching the walls will slightly reduce the aver age time of flight, and after a number of trips the oscillator will begin to oppose instead of aid. When this occurs, the electrons will not strike the cathode, but will oscillate in a constantly decreasing are within the cathode until they are picked up by the anode 29. It is from these collected out-of-phase electrons that part of the anode current is obtained.

The "electron drift" is also controllable by varying the gradient along the anode 29. Changing the value of resistor 29 changes the potential drop between plate I! and spring 2|, and hence changes the amount oi deflection during each electron passage across the tube. This varies the number of multiplication cycles, and hence the output current developed.

It should be noted that, while the anode potential may be many hundreds of volts, the potential supplied by the driving oscillator 3! need only be sufllcient to produce a secondary-generating impact, and may be of the order of 20 volts,

depending on'the magnitude of the anode potential and the characteristics of the particular emissive surface used.

Choke coil 34 prevents the oscillator from impressing its output on the signal output through lead 39, while permitting free passage of the signal current through resistor 35, and permitting battery 39 to maintaina bias on cathode 29 such that there will be no cut-'ofl' effect at the aperture 39 other than that exercised by grid 9 in accordance with the signal input.

The "electron drift principle is also incorporated in the multiplier shown in Figures 2 to 4, which is driven by direct current.

Figure 3 is a partially sectional view of the multiplier within an envelope 49 of glass or similar material. A reentrant stem 4! is extruded near one end of envelope 4!! and at right angles to the major axis thereof. A cathode, shown conventionally as a filament 42, is supported within the reentrant stem by leads 44 sealed therethrough. A control electrode, shown as an apertured shield 45, is supported by a connecting lead 46, also sealed through the reentrant stem, in such position that some of the electrons emitted by cathode 42 are permitted to pass into the main body of the envelope 49. Within envelope 40, the space between them constituting the multiplying chamber, are disposed two parallel plates 41 and 49, formed of material having high electrical resistance, and coated on the opposed surfaces with discrete particles of material having high secondary emission characteristics, such as caesium or caesium-silver oxide. The alignment of the plates 41 and 49 is maintained by supporting studs 59 and 5| sealed into envelope 40, and external connection is provided through a lead 52.

Midway between, and parallel to, plates 41 and 49, is a herringbone lattice or grid 54 of lowresistance wire, supported by two rods 55 and 56 of high-resistance material. The apices of the lattice wires 54 are directed toward the cathode end of plates 41 and 49, where an aperture 51, disposed through plate 41 in registry with the apertured control electrode 45, provides an entrance for electrons from filament 42 into the multiplying chamber. The grid-supporting rods 55 and 56 extend beyond the ends of plates 41 and 49 opposite aperture 51, terminating in a rectangular rim 59 formed at right angles to and integrally with the rods, the rim and entire grid assembly being supported by a lead 99 sealed through envelope 40. Across rim 59 is stretched a conducting grid 6|. A collecting anode 92, supported by a lead 54 sealed through the end of envelope 49, is disposed parallel to grid 9|.

Figure 4 shows a schematic sectional view, taken along line 4-4 of Figure 3, illustrating the manner in which the electrons moving between plates .41 and 49 are prevented from straying out of the multiplying chamber. Mica plates 19, apertured to permit the passage of grid wires 54, are used to enclose the space between plates 41 and 49.

The circuit diagram for operation of the embodiment of Figures 3 and 4 is shown in Figure 2. The filament 42 is heated by a source of energy not shown: of the emitted electrons, a number modulated by the input signal is admitted to the multiplying chamber through aperture 51. The control is exercised by apertured electrode 45, upon which a varying charge is impressed from condenser 65 by the signal'current in the input line 66. The end of the multiplying chamber nearest the cathode is connected to the negative side of a high-potential battery Bl, the positive side of which is applied to the collecting plate 62 through a resistor 69. In practice, the plate 62 may be about 600 volts positive with respect to the low-potential end of the multiplying chamber. Battery 51 is tapped at a point about 100 volts less positive than plate 62, and through lead 60 connected to the rectangular frame 59 and grid 6|. The high-resistance rods 55 and 56 are likewise tapped by a lead 10 at a point about 100 volts less positive than grid BI, and connected to the high-potential end of plates ti and 49. Opposite points on plates ll and 59 are maintained at equal potential by leads H and 12, connecting together the highand low-potential ends respectively of the two plates.

A constant difference of potential of 100 volts between opposite points on the plates Ail and t9, and the rods 55 and 56, is maintained by leads ill and 76. Lead It connects the low-potential ends of the rods 55 and 56 to plate 69 opposite a point on the rods 100 volts positive above the rod-ends. Lead 52 from the low-potential plate ends is returned to the negative side of battery Bl to complete the circuit.

The electrons admitted through aperture 57 are attracted toward the lattice grid 56, but due to its comparatively open character, the majority are not collected, and their momentum carries them on toward the plate 49. Since the admittance velocity is low, and the acceleration due to grid 55 is neutralized by the decelerating eifect once the electrons have passed through, the impact velocity would be too long to cause secondary emission were it not for the deflection due to the field gradient on the grid 54. The

effect of the gradient is intensified by the herringbone construction: the electrons are constrained to the longitudinal center of the multiplying chamber as well as deflected toward the high-potential end. In consequence of the iongitudinal deflection, the electrons will strike upon plate 59 at a point a number of volts more positive than was plate M at the aperture 51, and will retain sufiicient energy to knock secondaries out of the coating on plate 69.

These secondaries will in turn be attracted toward grid 56, deflected toward the high-potential end, pass through the grid, and strike plate 67 at a point still more positive than that on plate 39 from which they originated. When the electron stream reaches the high-potential end of the multiplying chamber, it is drawn toward grid 8i, passes through, and is collected upon anode plate 52. In returning to battery 61 from plate 62, a potential drop through resistor 69 is set up which is utilized to charge condenser I5 and send current through output lines 18 and TI.

The number of passages across the multiplying chamber, and hence the degree of amplification, is controlled by the potential gradient, and hence by the voltage of battery 61.

The similarity in the two embodiments presented lies in the use of an electron drift along, as well as across, a continuous emitting chamber, under the influence of a potential gradient set up in an intermediate electrode, the anode, in cooperation with an assisting potential on the chamber walls set up either by an external oscillator or another voltage gradient.

To those skilled in the art it .is obvious that the'form of all the elements involved may be varied within wide limits, as may the-spacing of the electrodes, the values of resistance and potential used, the material forming the emissive surfaces, the cathode embodiment, and so forth, without departing from the spirit and scope of the invention. The essential feature of the invention is the combination of means to produce an electron drift within the multiplier, whereby space current limitations may be removed, and the degree of multiplication controlled,

We claim:

1. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, opposed secondarily emissive cathode surfaces, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between said cathode surfaces, and means for producing potential gradients along said cathodes and said anode.

2. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, opposed secondarily emissive cathode surfaces, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between said cathode surfaces, means for setting up a positive potential on said anode relative to said cathode surfaces, and means for producing potential gradients along said cathodes and said anode.

3. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, opposed secondarily emissive cathode surfaces, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between said cathode surfaces, means for setting up a positive potential on said anode relative to said cathode surfaces, and means for producing equal potential gradients along said cathodes and said anode.

4. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, opposed secondarily emissive cathode surfaces, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between said cathode surfaces, means for setting up a positive potential on said anode relative to said cathode surfaces,

means for producing equal potential gradients along said cathodes and said anode, and means for maintaining opposite points on said cathode surfaces at equal potentials.

5. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, opposed secondarily emissive cathode surfaces, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between said cathode surfaces, means for charging said cathodes positively, means for setting up a positive potential on said anode relative to said cathode surfaces, means for producing equal potential gradients along said cathodes and said anode, and means for maintaining opposite points on said cathode surfaces at equal potentials.

6. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, parallel cathode plates formed of high-resistance material, a thin layer of discrete particles of secondarily emissive material having a low work function disposed upon the opposed surfaces of said plates, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between said cathode surfaces, means for setting up a positive potential on said anode relative to said cathode surfaces, and means for producing potential gradients along said cathodes and said anode.

7. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, parallel cathode plates formed of high-resistance material, a thin discontinuous layer of high secondary emissivity deposited upon the opposed surfaces of said plates, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between said cathode surfaces, means for setting up a positive potential on said anode relative to said cathode surfaces, and means for producing equal potential gradients along said cathodes and said anode.

8. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, parallel cathode plates formed of high-resistance material, a thin discontinuous layer of high secondary emissivity deposited upon the .opposed surfaces of said plates, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between said cathode surfaces, said anode comprising a pair of high resistance rods connected by a plurality of conductors, means for setting up a positive potential on said anode relative to said cathode surfaces, and means for producing equal potential gradients along said cathodes and said anode.

9. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, parallel cathode plates formed of high-resistance material, a thin discontinuous layer of high secondary emissivity deposited upon the opposed surfaces of said plates, the end of one of said plates being positioned to intercept said electron stream, an anode, positioned midway between said cathode plates, comprising a pair of high-resistance rods connected by a plurality of V-shaped conductors, the apices of said conductors being directed toward the end of said cathode plates at which said modulated beam is intercepted, means for setting up a positive potential on said anode relative to said cathode surfaces, and means for producing equal potential gradients along said cathodes and said anode.

10. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, opposed secondarily emissive cathode surfaces, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between said cathode surfaces, means for producing potential gradients along said cathodes and said anode, means for diverting electrons from between said cathode surfaces at the most positively charged portions thereof, and means for collecting said diverted electrons.

11. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, parallel cathode plates formed of high-resistance material, a thin discontinuous layer of high secondary emissivity deposited upon the opposed surfaces of said plates, an anode comprising a pair of highresistance rods connected by a plurality of conductors, an anode ring formed normal to and integral with said rods, a plurality of conductors positioned across said ring, a collecting plate disposed parallel to said anode ring, and separate leads from said electrodes sealed through said envelope.

12. An electron multiplier comprising an evacuated envelope having therein means for producing an electron stream, parallel cathode plates formed of high-resistance material, a thin discontinuous layer of high secondary emissivity deposited upon the opposed surfaces of said plates, one of said surfaces being positioned to intercept said electron stream, a perforate anode positioned parallel to and midway between saidcathode surfaces, said anode comprising a pair of high-resistance rods connected by a plurality of V-shaped conductors having the apices thereof directed toward the point of interception of said modulated beam, said rods terminating at the ends opposite said beam interception point in a rectangular rim formed homogeneously with said rods and normal thereto, a conducting grid disposed across said rim, a collecting plate positioned in registry with said rim, means for setting up a positive potential on said anode relative to said cathode surfaces, means for producing equal potential gradients along said cathodes and said anode, means for setting up a positive charge on said grid relative to the most positive of said V-shaped conductors, and means for charging said collecting plate positively with respect to said grid.

13. In an electron multiplier having an anode and secondarily emissive cathode surfaces arranged to be serially impacted, means for producing a unidirectional electron drift parallel to said cathode surfaces between successive cathode impacts, including means for setting up a potential gradient along said anode parallel to said cathode surfaces.

PHILO T. FARNSWORTH. RICHARD L. SNYDER. 

